1. Field of the Invention
The present invention relates to a semiconductor switch circuit equipped with a high breakdown voltage bidirectional analog switch, and a signal processing apparatus and an ultrasound diagnostic apparatus using the semiconductor switch circuit.
2. Description of the Related Art
There has recently been a demand for further miniaturization and decreasing in costs for an ultrasound diagnostic apparatus with its wider use. The ultrasound diagnostic apparatus has a piezoelectric probe for transmitting and receiving ultrasound, and a bidirectional analog switch for transmitting and receiving a drive signal of the piezoelectric probe and a signal of the ultrasound and further includes a plurality of power supplies for driving the bidirectional analog switch. A current ultrasound diagnostic apparatus is accompanied by a problem that the mounting of a plurality of power supplies becomes a bottleneck and a drastic size reduction cannot be realized.
There has been described in the problem of JP-2012-95168-A that “there is provided a semiconductor device of a bidirectional analog switch having satisfactory linearity and less reduced in power loss, and there is provided an ultrasound diagnostic apparatus high in detection precision”. There has been described in its solving means that “there is provided a semiconductor device of a bidirectional analog switch incorporating a switch circuit capable of turning-on or off bidirectionally and a drive circuit for the switch circuit therein, wherein the drive circuit is connected to first and second power supplies, the first power supply voltage is greater than or equal to the maximum voltage value of a signal applied to an input/output terminal of the switch circuit, and the second power supply voltage is less than or equal to the minimum voltage value of the signal applied to the input/output terminal of the switch circuit, and further, wherein the drive circuit has zener diodes and P type MOSFETs connected in series between the first power supply and the switch circuit. There is provided an ultrasound diagnostic apparatus equipped with the semiconductor device”.
An essential configuration of an ultrasound diagnostic apparatus 2A according to a comparative example is shown in FIG. 5.
As shown in FIG. 5, the ultrasound diagnostic apparatus 2A has a transmission drive unit 7 that generates a transmission signal, based on a trigger signal, an electronic switch 9A that supplies the transmission signal to a probe 4, the probe 4 comprised of a plurality of piezoelectric elements (not shown), a driving power supply 5 that supplies a variable voltage, and a bias power supply 3 that supplies a fixed voltage. The ultrasound diagnostic apparatus 2A performs switching operations of a plurality of bidirectional analog switches lying inside the electronic switch 9A and supplies the transmission signal to the prescribed piezoelectric element of the probe 4 to thereby perform ultrasound scanning.
The electronic switch 9A has semiconductor switch circuits being the plural bidirectional analog switches in association with the plural piezoelectric elements provided in the probe 4. AS compared with a mechanical relay, the semiconductor switch circuit has various advantages such as high reliability, a reduction in size, high-speed switching, low power consumption, a noise reduction, a long lifetime, etc.
The driving power supply 5 generates positive and negative drive voltages, based on a voltage control signal in response to the supply of AC power (described as an AC input in the figure) and supplies the generated positive and negative drive voltages to the transmission drive unit 7.
The bias power supply 3 generates fixed positive and negative bias voltages in response to the supply of the AC power and supplies the generated positive and negative bias voltages to the electronic switch 9A. The positive and negative bias voltages are voltages higher than the positive and negative drive voltages supplied from the driving power supply 5.
Thus, the ultrasound diagnostic apparatus 2A has both the driving power supply 5 that uniquely generates the drive voltages necessary for the operation of the transmission drive unit 7, and the bias power supply 3 that uniquely generates the bias voltages necessary for the control of the electronic switch 9A. Consequently, the ultrasound diagnostic apparatus 2A has caused an increase in the number of components and an inhibition factor of miniaturization.
FIG. 6 shows a block diagram of a high breakdown voltage analog switch integrated circuit 90A according to a comparative example.
The high breakdown voltage analog switch integrated circuit 90A (one example of semiconductor switch circuits) according to the comparative example, which configures an electronic switch 9A, has a shift register 91 that performs operation switching of a plurality of switches, a latch circuit 92 that holds the operation states of the switches, an output switch 8A, and a level shift control circuit 1A that drives the output switch 8A.
The shift register 91 outputs an input signal DIN to each stage of the latch circuit 92 as a signal D while being shifted in synchronization with an input signal CLK. An output signal DOUT is outputted as a signal outputted to the last stage of the latch circuit 92.
The latch circuit 92 is configured in a multiple stage and latches the signal D and a signal CL therein in accordance with an input signal LE. The latch circuit 92 outputs the latched signals to respective stages of the level shift control circuit 1A.
The level shift control circuit 1A is configured in a multiple stage and turns ON/OFF respective stages of the output switch 8A, based on the signals latched by the latch circuit 92.
The output switch 8A is configured in a multiple stage. The first stage of the output switch 8A performs switching as to whether or not to transmit an analog voltage signal between an input/output terminal 101-1 and an input/output terminal 102-1. Subsequently, in the same manner as above, the n-th stage of the output switch 8A performs switching as to whether or not to transmit an analog voltage signal between an input/output terminal 101-n and an input/output terminal 102-n. 
The shift register 91 and the latch circuit 92 constitute a low voltage circuit with a low voltage power supply VDD as a drive power supply. The output switch 8A and the level shift control circuit 1A constitute a high voltage circuit with a positive high voltage power supply VPP and a negative high voltage power supply VNN as drive power supplies. The positive high voltage power supply VPP is set higher than the signal voltage applied to each of the input/output terminals 101 and 102. The negative high voltage power supply VNN is set lower than the signal voltage applied to each of the input/output terminals 101 and 102.
FIG. 7 shows a circuit diagram of part of the high breakdown voltage analog switch integrated circuit 90A according to the comparative example. FIG. 7 shows only one stage of the output switch 8A that serves as the essential part of the high breakdown voltage analog switch integrated circuit 90A, and the level shift control circuit 1A that serves as its drive circuit.
As shown in FIG. 7, the output switch 8A has a switch pair 85 comprised of two MOSFETs (Metal-Oxide-Semiconductor Field-Effect Transistors) 851 and 852 of which the gates and sources are each connected to one another, and a switch unit 86 comprised of a MOSFET 861.
The MOSFET 851 has a drain connected to the input/output terminal 101. The MOSFET 852 has a drain connected to the input/output terminal 102. The MOSFET 861 has a source connected to the negative high voltage power supply VNN and a drain connected to a connecting node of sources of the MOSFETs 851 and 852.
The level shift control circuit 1A supplied with the positive high voltage power supply VPP and the negative high voltage power supply VNN as drive power supplies controls the output switch 8A on the basis of a switch control signal SCTL.
The switch pair 85 is an analog switch that performs the transmission and cut off of a high voltage signal between the input/output terminal 101 and the input/output terminal 102. The switch unit 86 acts as a shunt for improving off isolation performance indicative of signal cut off performance when the analog switch is turned off.
When the high voltage signal is applied to the input/output terminals 101 and 102, a high voltage is applied between the gates and sources of the switch pair 85. Therefore, the MOSFETs 851 and 852 that configure the switch pair 85 need to have a gate structure of which is a high breakdown voltage. Since the MOSFETs 851 and 852 of which the gates have such a high breakdown voltage are high in on voltage, the level shift control circuit 1A that drives them also needs to be supplied with the high voltage power supply.